1. Field of the Invention
The present invention relates to a bipolar, filter press type electrolytic cell. More particularly, the present invention is concerned with a bipolar, filter press type electrolytic cell for the production of chlorine and an alkali metal hydroxide by electrolyzing an aqueous alkali metal chloride solution. The electrolytic cell comprises a plurality of unit cells which are arranged in series through a cation exchange membrane disposed between respective adjacent unit cells, each unit cell containing anode-side and cathode-side gas-liquid separation chambers respectively disposed in anode-side and cathode-side non-current-flowing spaces and extending over the entire upper-side lengths of anode and cathode compartments. The filter press type electrolytic cell of the present invention can be utilized to stably perform the electrolysis of an aqueous alkali metal chloride solution at a low cost and with great advantages in that not only does leakage of an electrolytic solution not occur, but a good circulation of the electrolytic solution within the anode and cathode compartments is also assured over a wide range of the internal pressure of the cell. Also a vibration of the cell and formation of a gas zone in the upper portion of each of the anode and cathode compartments are effectively prevented even at a high current density and at a high alkali concentration, so that occurrence of breakage of and pinhole formation in the ion exchange membrane can be effectively prevented.
2. Discussion of Related Art
Many proposals have heretofore been made with respect to the process for the electrolysis (hereinafter frequently referred to as "ion exchange membrane method electrolysis") of an alkali metal chloride using an ion exchange membrane for the production of a high purity alkali metal hydroxide at high current efficiency. For example, U.S. Pat. No. 4,108,742 discloses a method in which the electrolysis is conducted while maintaining the internal pressure of the cathode compartment at a level higher than the internal pressure of the anode compartment; Japanese Patent Application Laid-Open Specification No. 51-103099 discloses a method in which a mineral acid is incorporated into an anolyte and the electrolysis is conducted while maintaining at 3.5 or less a pH value of the saline solution present in the anode compartment; U.S. Pat. No. 4,105,515 discloses a method in which the electrolysis is conducted while maintaining the pressures of a halogen gas in the anode compartment and a hydrogen gas in the cathode compartment at a superatmospheric pressure; and U.S. Pat. No. 4,214,957 discloses a method in which the electrolysis is conducted while a fresh saline solution to be supplied and/or a low concentration saline solution to be recycled are allowed to absorb hydrogen chloride gas. These methods are effective for lowering an electrolysis voltage or decreasing the oxygen concentration of an evolved chlorine gas. However, these methods are not satisfactory from the viewpoint of effectively conducting the electrolysis without formation of a gas zone in the anode and cathode compartments even at a high current density while preventing the vibration of the cell. The vibration of the cell leads to breakage of an ion exchange membrane. Further, as will be described later in detail, formation of a gas zone in the anode and cathode compartments leads to pinhole formation in and breakage of an ion exchange membrane.
With respect to the conventionally proposed electrolytic cells for the electrolysis of an alkali chloride, reference can be made, for example, to U.S. Pat. No. 4,111,779 in which an electrical connection between anode and cathode compartments in a unit cell is established by spot welding through an explosion-bonded titanium-iron plate; U.S. Pat. No. 4,108,752 in which an electrical connection between anode and cathode compartments in a unit cell is established by means of a spring type connector; Canadian Patent No. 1076994 in which an electrolytic cell is made from a plastic, and an electrical connection between anode and cathode compartments in a unit cell is established by means of bolts and nuts; and Japanese Patent Application Laid-Open Specification No. 54-90079 in which an electrical connection between anode and cathode compartments in a unit cell is established by bonding titanium as a material of an anode-side partition wall and stainless steel as a material of a cathode-side partition wall through a copper plate by ultrasonic welding. These conventional electrolytic cells are improved in the construction of electrolytic cells and in the reduction of electrical resistance between anode and cathode compartments in a unit cell. However, in these conventional electrolytic cells, no special consideration is given for solving the problems which are encountered when electrolysis is conducted at a high current density, i.e., the problems of vibration of the cell, occurrence of uneven concentration distribution of an electrolyte (solute) within electrode compartments and formation of a gas zone in the upper portion of the electrode compartments. The unevenness in the concentration of an electrolyte is caused by poor circulation of the electrolytic solution, and is likely to adversely affect the desired performance of an ion exchange membrane.
In U.S. Pat. No. 4,557,816, a duct is provided in electrode compartments to thereby improve the uniformity of the electrolyte concentration in the electrode compartments, but there are drawbacks in that vibration of the cell and formation of a gas zone in the upper portion of the electrode compartments occur when electrolysis is conducted at a high current density.
On the other hand, U.S. Pat. No. 4,643,818 discloses an electrolytic cell which can be used as either of a monopolar type cell and a bipolar type cell, and U.S. Pat. No. 4,734,180 (corresponding to EP No. 0 220 659 B1) discloses an electrolytic cell in which each unit cell is provided by disposing an anode-side pan-shaped body and a cathode-side pan-shaped body back to back, each pan-shaped body comprising a partition wall, a frame wall extending from the periphery of the partition wall and upper and lower hooked flanges, respectively, extending from the upper-side and lower-side portions of the frame wall, and fittedly inserting an upper and lower engaging bars, respectively, into upper and lower through-spaces which are, respectively, formed between the upper-side portions of the frame wall and the upper hooked portions and between the lower-side portions of the frame wall and the lower hooked portions when both pan-shaped bodies are disposed and fastened back to back. The above-mentioned two U.S. patents are advantageous in that not only can the number of welded portions be reduced and no leakage of an electrolytic solution occurs even at a high internal pressure of the cell, but also the assembling of each unit cell can be conducted easily and at low cost. However, the electrolytic cells of the above U.S. patents are unsatisfactory with respect to the circulation of an electrolytic solution within electrode compartments and to the prevention of formation of gas zone and of vibration of the cell when it is desired to stably conduct electrolysis under operation conditions such that the internal pressure varies over a wide range from a superatmospheric pressure to a reduced pressure or when it is desired to stably conduct electrolysis at a current density as high as 45 A/dm.sup.2 or more.
Further, Japanese Patent Application Laid-Open Specification No. 61-19789 and U.S. Pat. No. 4,295,953 disclose an electrolytic cell in which a cell frame has a hollow structure and is of a picture frame-like shape, and an electrically conductive spacer is disposed between an electrode plate and an electrode sheet, the spacer being intended to serve as a path for the downward flow of an electrolytic solution. Japanese Patent Application Laid-Open Specification No. 63-11686 discloses an electrolytic cell in which a cell frame has a hollow structure and is of a picture frame-like shape, and a cylindrical member for electrical current distribution is provided, the cylindrical member being intended to serve as a path for the downward flow of an electrolytic solution. In these prior art techniques, an improved circulation of an electrolytic solution in electrode compartments can be attained, but when electrolysis is conducted at a high current density, it is likely that vibration occurs around an outlet for liquid and gas and that a gas zone is formed in the upper portion of the electrode compartments. Further, in these techniques, disadvantages are likely to be encountered such that when it is attempted to increase the internal pressure of the cell, the strength of the cell is unsatisfactory; that a leakage of an electrolytic solution occurs; and that when it is attempted to conduct electrolysis while adding hydrochloric acid into a fresh electrolytic solution (in order to prevent an increase in the oxygen concentration of evolved chlorine gas and prevent formation of chlorate), the voltage of the ion exchange membrane is increased.
Thus, although many conventional techniques were proposed for effectively and efficiently conducting the ion exchange membrane method electrolysis of an alkali metal chloride, no conventional proposal is satisfactory in meeting the recent demand for the prevention of occurrence of vibration of the cell during the electrolysis and demand for the capability of conducting electrolysis at an advantageously low voltage even at a current density as high as 45 A/dm.sup.2 or more, i.e., demand for high efficiency, power consumption saving and the like.